This invention relates to measurement of surface emissivities and in particular to a method and apparatus for independently determining the electromagnetic surface emissivity of a material.
Knowing the electromagnetic emissivity of reflective surfaces is important for applications in radiometry, both on the ground and in space, and especially for communication systems working at extremely high frequencies in space. For such applications, it is assumed that the absorptivity of these surfaces is equal to the emissivity. In the case of large deployable antennas in space employing mesh membrane surfaces, a large emissivity could introduce error effects due to temperature variations in space thereby making radio reception questionable or highly inaccurate. Consequently, the surface emissivity of rigid or large deployable antennas using mesh membrane material must be determined accurately so that emissivity can be decreased to zero by means of proper selection of surface coating and wire or rib spacing per unit area.
In the past, measurement of the electromagnetic properties of deployable reflectors have been achieved using active systems in anechoic chambers, i.e., the power of a transmitting horn antenna is reflected off the surface under test and the reflected signal is received by another horn antenna. First, the ratio of the received power to the transmitted power of a near perfect reflector is determined and, secondly, the equivalent power ratio of the mesh under test is measured. By comparing the two ratios, it is possible to deduce the sum of the emissivity losses plus the transmission losses. This procedure has the disadvantage that a separation of the emissivity losses from the sum of the losses is impossible. Further, since the emissivity loss is the dominating generator of white-noise and since in the determination of the white-noise, the emissivity is multiplied by its physical temperature according to a derivation of the Raleigh-Jeans approximation, the emissivity generates uncorrelated electromagnetic energy due to temperature variation in the mesh. This energy can be so high that a radiometer measurement either of the Earth's surface properties or of the universe are very inaccurate especially in cases of low contrast with the background. The interference from thermally induced noise becomes even more degrading to the measurement of the aforementioned properties when the physical temperatures across a large antenna (100 m) are not uniform and change along each orbit in space because of the sun's radiation.
The radioastronomy community evaluates the quality of its measurements in the universe by determining the G/T parameter (gain over input noise temperature) which defines the effects of the intervening atmosphere and the receiver noise contribution. However, for the design of large space antennas, the electromagnetic characteristics of surfaces, especially the emissivity, must be known before the planning of space programs and space structure designs. The G/T method is not applicable for such design work because the emissivity is lost in all of the other effects.
Accordingly, it is an object of the present invention to accurately determine the electromagnetic surface emissivity of a mesh membrane material of the type used in large deployable antennas.
It is a further object of the present invention to accurately determine the electromagnetic surface emissivity of any radiating material in order that the material may be manufactured so as to decrease the emissivity to zero thereby reducing the emissivity energy losses of the material.
A still further object of the present invention is to accurately determine the electromagnetic surface emissivity losses of a material independent of any other surface losses.
Other objects and advantages of the present invention will be readily apparent from the following description and drawings which illustrate a preferred embodiment of the present invention.